Page 188 - Multidimensional Chromatography

Multidimensional Planar Chromatography 181

Figure 8.10 Separation of polyphenolic compounds by BMD separation.

Bivariate multiple development (BMD) with a mobile phase gradient of decreas-
ing S T is preferable for the analysis of less complex mixtures of wide polarity,
because the complete chromatographic separation finally obtained can now be
detected as a single chromatogram. In BMD, the first development is performed over
the shortest distance by using the mobile phase system with the greatest S T ; the
development distance is increased and the S T is then reduced with each subsequent
development until the final development is performed over the longest distance with
the weakest mobile phase. By use of BMD with a mobile phase gradient of decreas-
ing polarity we have separated (18) polyphenolic compounds spanning a wide polar-
ity range, in two chromatographic steps (Figure 8.10).
In the first development step, the more polar components were separated by a
60 mm chromatographic development with a mobile phase of high S T ; the polar
compounds were distributed between the origin and the solvent front of the first
chromatographic step (18). The moderately polar compounds were transported close
to the solvent front by this mobile phase. After complete drying of the plate, in the
second chromatographic step the layer was developed for a greater distance (80 mm)
with a mobile phase optimized for the separation of compounds of lower polarity.
This second mobile phase with a lower S T does not significantly affect the position of
polar components eluted in the first development step, but resolves the compounds
of lower polarity between the fronts of the first and second development steps.
The basis of automated multiple development (AMD) is the use of different
modes of multiple development in which the mobile phase composition (S T and S V
values) is changed after each, or several, of the development steps. Figure 8.11 illus-
trates the principle of AMD employing a negative solvent-strength gradient (decreas-
ing S T values).
In the first step, the compounds to be separated were moved and reconcentrated.
In subsequent steps, both values characterizing the mobile phase were changed